Heini Reijonen writes about the use of geological investigations in relation to the geological disposal of radioactive waste. Projects of this type have been conducted at GTK for a long time, and the work is also currently ongoing. Here, a brief introduction is given to the whole concept of natural analogues and demonstrating the long-term safety of disposal facilities for nuclear waste. Natural analogues are actively being discussed in the scientific community, and the development of new projects is ongoing. More information on natural analogues can be found through the references below and at www.natural-analogues.com.

Geological disposal of radioactive waste

Several options have been investigated for the management of radioactive waste, especially high-level wastes, such as spent nuclear fuel. Geological disposal is the internationally preferred option to deal with high-level radioactive waste. Even in cases where the fuel is treated or re-used, high-level waste, with variable compositions, will still be produced.

The geological disposal of nuclear waste is based on natural (repository host rock) and engineered (man-made) barriers that prevent or retard the potential release of radioactive substances to the biosphere (see e.g. Posiva 2012, chapter 4, for more discussion).

Engineered barrier systems (EBS) can include various materials, such as glass, ceramics, steel, copper, titanium, clays (especially bentonite), cements, concrete and various types of mixtures of rock materials (and this list is by no means exhaustive), that also help to also minimise the release of radionuclides from the waste or are part of the repository construction. Repository designs vary from country to country, depending on the waste types and host rock.

Isolation of the waste from the environment is provided by sufficient depth in the repository host rock and the overall repository design. The decay process will reduce the radioactivity of the waste over time, but since this takes a long time from the human perspective, the long-term behaviour of the system needs to be studied.

The long-term safety of geological repositories is assessed through a safety case, where all knowledge and analyses performed to test the repository performance and safety are presented and summarised. In this exercise, the processes taking place in the repository need to be assessed over geological time scales. Extensive site investigations together with laboratory and modelling studies are used to understand the performance of the repository in the long term. Natural analogues (NA) can be used to understand long-term processes in repository environments.

What are natural analogues?

A NA study can be any form of investigation of any relevant natural system, as long as it provides quantitative or qualitative information that can be used to support (and build confidence in) geological disposal (Alexander et al. 2015). This may mean that a study provides data that are directly applicable to the safety case (e.g. the corrosion rate of a metal used in the waste packaging) or, alternatively, it may provide illustrations of concepts or processes that can demonstrate safety (e.g. the overall properties of materials such as bentonite, often occurring as a hydrological barrier in sedimentary strata).

Long-term safety and the underlying science

The long-term safety assessment period for a geological repository covers time scales of hundreds of thousands to millions of years. Short-term information obtained from laboratory experiments (e.g. radionuclide transport over a few millimetres), underground rock laboratory (URL) experiments (e.g. bentonite swelling behaviour over a few years) and from industrial analogues (e.g. cement durability information from centuries-old constructions) can be used to understand mechanisms and processes. However, for a long-term assessment, additional arguments are required that rely on evidence gained from natural and archaeological systems that have operated over the temporal (i.e. thousands of years to geological timescales) and spatial (i.e. tens of metres to kilometres) scales of relevance to a repository (Figure 1). This has been accepted by the scientific community to the extent that the use of natural analogues to support a safety case is even enshrined in national regulation in several countries (e.g. STUK 2014). How and what types of natural analogues can be used in the long-term assessment of performance depend on the stage of the geological repository project (Table 1). For further reading, see Reijonen et al. (in prep).

Figure 1. Schematic illustration of the differing spatial and temporal scales associated with different data sources in relation to two repository relevant bentonite processes (from Savage 2011).Table 1. Using information from a natural analogue in different stages of the disposal project (simplified from Reijonen et al. in prep.).

Applied geosciences!

As in many other safety evaluations worldwide, natural analogues have also been discussed at length in the Finnish repository project (for Posiva’s 2012 safety case, see Posiva (2012) and Reijonen et al. (2015); for the repository site on Olkiluoto Island in Finland (Figure 2). The same processes that occur in repository environments in relation to the behaviour of, for example, copper, iron, clay, cement and (radioactive) waste matrices can be observed by looking at natural systems. A fair amount of work on natural analogues has been carried out over the past few decades (see e.g. Alexander et al. 2015 and references therein), but a lot of potential still exists to obtain increasingly case-specific data through geological investigations. As a geological survey, we have lot to offer, since natural analogues are essentially about geology and understanding long-term processes. GTK has recently participated in several projects focusing on natural analogues, one of the most important being the Greenland Analogue Project (GAP), concentrating on the processes occurring at the margins of a continental ice sheet (Claeson Liljedahl et al. 2017). A long-term project on post-glacial faults has also been ongoing for a few years, with lots of interesting new results.

Natural analogues can be used to provide data and help to understand the likely processes occurring in the future, but they also provide a useful means to discuss with various stakeholders. Here, research can provide added value at several levels of detail, with technical information based on hard science, but simplified examples can also be extracted to better explain (complex processes) to various groups of interested people.

Figure 2. Olkiluoto Island and a schematic representation of the repository for spent nuclear fuel, showing details of the copper canisters in disposal holes surrounded by a bentonite buffer (Source: media library of Posiva).

References

Below, references are given to the publications mentioned above and some other recent publications by the author that discuss various aspects of this interesting branch of applied geology.

Heini Reijonen works as a chief expert in the Bedrock Construction and Site Assessment unit of the Geological Survey of Finland. She has a decade of experience in the long-term safety assessment of the geological disposal of radioactive waste and has been involved in research projects focusing especially on geological aspects of bentonites.